Copper alloys for electronic materials used in various electronic components such as connectors, switches, relays, pins, terminals, lead frames and the like are required to satisfy both of high strength and high electrical conductivity (or thermal conductivity) as fundamental properties. In recent years, high integration, miniaturization and reduction of thickness of electronic components are rapidly progressing and correspondingly the requested level for the copper alloys used in the components for these electronic devices has been becoming higher and higher.
From the aspects of high strength and high electrical conductivity, the use of precipitation-hardened copper alloy as copper alloy for electronic materials is increasing in amount, in place of the conventional solid solution-strengthened type alloys represented by phosphor bronze, brass or the like. With respect to the precipitation-hardened copper alloy, a supersaturated solid solution, which has been subjected to solution treatment, is subjected to ageing treatment, whereby fine precipitates are homogeneously dispersed and not only the strength but also the electrical conductivity of the alloy are increased, because of the decreased amount of solid solution elements in the copper. For this reason, a material which excels not only in the mechanical strength of the alloy such as strength and resilience but also in the electrical conductivity and thermal conductivity can be obtained.
Among the precipitation-hardened copper alloys, Cu—Ni—Si-based copper alloy (generally called Corson alloy), is one of typical copper alloys which have a relatively high electrical conductivity, a high mechanical strength and a high bending workability and is currently being actively developed in the industries concerned. With this copper alloy, the strength and the electrical conductivity are both improved by precipitating fine particles of Ni—Si-based intermetallic compound in the copper matrix.
Recently, an attempt of improving the properties of Cu—Si—Co-based copper alloy instead of the Cu—Ni—Si-based copper alloy is underway. For example, Japanese Patent Application Publication No. 2010-236071 (Patent Literature 1) discloses, for the purpose of obtaining a Cu—Si—Co-based alloy having superior mechanical and electrical properties as well as mechanical homogeneity, a copper alloy containing 0.5-4.0 mass % of Co, 0.1-1.2 mass % of Si and the balance Cu and unavoidable impurities, wherein the average grain size is 15-30 μm, and the average difference between the maximum grain size and the minimum grain size per each field of view of 0.5 mm2 is 10 μm or less.
The process of producing copper alloy disclosed in the patent document comprises the following sequential steps:                step 1 of melt-casting an ingot having a desired composition;        step 2 of heating the ingot to 950-1050° C. for at least one hour and thereafter subjecting it to hot rolling, setting the temperature at the time of completion of the hot rolling to at least 850° C., and cooling it from 850° C. to 400° C. at an average cooling rate of at least 15° C./sec;        step 3 of cold rolling with a working ratio of at least 70%;        step 4 of aging treatment at 350-500° C. for 1-24 hours;        step 5 of performing solution treatment at 950-1050° C., and then cooling the material temperature with an average cooling rate of at least 15° C./sec from 850° C. to 400° C.;        optional step 6 of cold rolling;        step 7 of ageing treatment; and        optional step 8 of cold rolling.        